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Tek 2465 vertical calibration? Hardware problem? User error?
i386:
I have a 2465 (not A or B) that I bought years ago and never really used or knew how to use it. Recently I dragged it out of storage and it blew one of the capacitors. I replaced all of the Rifa caps and one carbon comp resistor that was clearly damaged in the blast. The scope starts up with no errors and I’ve spent a great deal of time in the operators and service manual as well as watching hours of videos. I’m now pretty familiar with the scope’s controls. I also recently scored a BK precision function generator.
What I’m noticing is when measuring voltages on Channel 2 is they read exactly 5x higher on Channel 2 than on Channels 1, 3, or 5.
I have a 50ohm BNC cable connecting my signal generator to my scope inputs. Starting on channel one I set the frequency to 1Khz and the amplitude so that the scope measures it at .1V. Channel 2 will measure that same signal as .5V. I have my V/Div set the same on all 4 channels. I’m triggering on whichever channel the cable is plugged into. All 4 channels measure the frequency correctly as 1Khz.
It seems odd to me that it’s off exactly by a factor of 5. I’m not sure if the vertical calibration is off, there’s a hardware problem, user error or what. Everything else is functioning normally as far as I can tell. I’ve read through the CAL02 procedure several times but haven’t mess with that yet.
I’m using this for tube amp work. I’m mainly interested observing clipping in my sine wave signal and measuring voltages at various points in the signal path. I will rarely have a need for channel 2 anyway. Still I have the itch to fix things when they don’t work.
i386:
Update I found a post on the tekscopes group where a guy had a similar problem. His was more intermittent and changed when he fiddled with the V/Div knob but he had the same 5x voltage problem. Turns out he was able to fix it by cleaning the contacts in the attenuator. I think I’m in the right neighborhood. I should have time to work on it this weekend.
bdunham7:
You should try it with a few different amplitude signals and some different V/div settings to see if it is consistent or changes. It likely will change and it probably is an attenuator issue. The attenuators on those are pretty delicate and fiddly, so tread carefully. If it becomes hopeless, let me know as I have a few parts mules packed away somewhere.
i386:
--- Quote from: bdunham7 on November 27, 2024, 04:48:43 am ---You should try it with a few different amplitude signals and some different V/div settings to see if it is consistent or changes. It likely will change and it probably is an attenuator issue. The attenuators on those are pretty delicate and fiddly, so tread carefully. If it becomes hopeless, let me know as I have a few parts mules packed away somewhere.
--- End quote ---
Thanks, I checked something else. At the appropriate coupling settings I’m able to measure 1M Ohm and 50 Ohms through the attenuator on Channel 1. On Channel 2 where I should be seeing 1M Ohm, I have an open circuit and where it should read 50 Ohms I’m getting fluctuating readings (and none of them are 50 Ohms.)
The service manual recommends replacing the attenuator if it’s bad so no cleaning instructions there. However, I read on the tekscopes group that they are “quite serviceable”. I’ve seen some pics of it disassembled and yes it looks pretty delicate in there. I’ll try to find an article or video of someone cleaning it so I’m not just going in blind. This old scope is in great shape. I’d love to see it working 100% again.
bdunham7:
That does look pretty mint! IDK if you've already heard this warning, but you should never run that scope with the cover off unless you have a fan blowing on the underside circuit board (you'll typically have the scope on its side or propped up) and if you end up having to disassemble or repair the cooling fan you should read the instructions at least 3X.
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